1. Field of the Invention
The present invention relates generally to a wireless communication system and time synchronization method thereof and, in particular, to a wireless communication system including base stations and Remote Radio Headers (RRHs) equipped with Global Positioning System (GPS) receivers and a method for acquiring and maintaining GPS synchronization in the wireless communication system.
2. Description of the Related Art
A base station uses GPS signals received from GPS satellites in order to acquire synchronization with a network. The GPS signal strength is comparable to the visible strength of a fog light of a vehicle from a distance of about 16000 km, under the assumption that the surface of the earth is flat. Accordingly, if the line of sight between a GPS receiver and the GPS satellite is not guaranteed securely, the GPS receiver is likely to fail in receiving the GPS signal. Since the GPS signal is very weak, if other strong signals on similar frequency bands exist, the GPS signal receiver is also likely to fail receiving the GPS signal.
If the GPS receiver fails to interpret the GPS signal correctly, the base station is also likely to malfunction or operate in limited capability. In a worst-case scenario, the GPS signal reception failure may result in a termination of communication. Due to the diversification of radio technologies, the GPS frequency band is being increasingly overlapped with other communication or broadcast system frequency bands. This overlapping makes it even more difficult to secure reliable GPS signal reception environment, and, as this overlapping accelerates, it is expected the situation will worsen even more rapidly.
In order to minimize the damage caused by GPS signal reception performance degradation, many studies are being conducted. One example of a method for reducing such damage is to integrate the GPS receiver in a digital block of the base station and use a high accuracy local oscillator in order to provide the GPS receiver with a holdover function.
In the wireless communication system, a network synchronization block equipped with a GPS receiver is very important in view of absolute time synchronization of the entire network. Accordingly, the base station is provided with at least two network synchronization blocks responsible for the same function at specific parts with the application of clock duplication. Since each of the network synchronization blocks have its own respective GPS antennas, such a configuration causes a tradeoff between system capacity and installation space and increases cost.
Although the clock duplication is used, it is necessary to use the high accuracy local oscillator in order to compensate for GPS signal loss caused by environmental change and device malfunction (i.e. for providing a holdover function). In such applications a high accuracy local oscillator is a very sensitive and expensive device. Recent high accuracy local oscillators are small enough for such applications, but there is also a preference for special oscillators that overcome ambient temperature sensitivity. Also, current systems implemented based on the local oscillator are limited in that the use of relatively large and expensive local oscillators causes spatial restrictions and increases manufacturing costs. Therefore, there is a need for an improved time synchronization method for a wireless communication system.